This invention concerns a method and apparatus for controlling the rotational speed and phase of synchronous motors. The invention includes a rotor with at least one pair of poles, and a stator with at least one field coil that is charged with driving pulses. The invention is specifically applicable with reaction motors of time-keeping devices such as clocks, using a pulse generator, which generates pulses of constant frequency and width.
In the case of synchronous motors, especially in the case of motors in the area of fine mechanics, it is frequently necessary to adhere to the most constant possible rotational speed and/or the most constant possible number of revolutions in a given time interval. With pulse-driven synchronous motors, it is also desirable to keep as constant as possible the phase of the rotor poles, with respect to the pulses with which the field coil is being charged. The reason for this is that too severe a phase shift frequently is the beginning of a permanent deviation of the rotational speed. This happens, for example, when the driving pulses "overtake" the poles of a rotor by integer multiples, a process which is called a pole jump.
Deviations with respect to the rotational speed and phase are based both on internal and external influences. Among these are different loading moments, frictional forces, and inertial forces, which can act on a rotating system with an appropriate moment of inertia. The last-mentioned case is particularly prevalent with transportable clocks and among these, particularly with wristwatches. If the driving system is sensitive to shock, this can lead to permanent status deviations, which can accumulate, in the course of time, to form intolerable indicator errors. Such influences can be counteracted by designing the motor and the control appropriately. However, this entails increased power consumption on the part of the motor. If the motor is driven by batteries, this leads either to a frequent replacement of batteries, or to batteries of very large size. Both contingencies are especially undesirable with watches. Batteries with a large volume are intolerable with wristwatches, especially with ladies' watches.
The prior art includes reactive synchronous motors with at least one field coil, where the field coil is charged with an alternating voltage. This alternating voltage is synchronous with the rotational motion of the magnetic field that is generated by the rotor. Such a motor not only consumes much power, but also has the disadvantage that a pulse that has been lost through a pole jump can no longer be retrieved. Such a system cannot keep constant the number of revolutions in a prescribed time interval.
DE-OS No. 23 05 682 discloses that a watch drive can be charged with two pulse trains, that differ slightly with respect to frequency, but that have the same pulse width. One of these pulse trains has a lesser frequency and the other has a greater frequency than would be theoretically required for absolute running accuracy. The relative switch-on time of the two pulse trains is controlled and differs over a longer time interval, and in this way the driving speed oscillates about an average value. Inasmuch as a drive by a synchronous motor is mentioned, the rotational speed of the motor is supposed to be changed by changing the energy supply. The idea deals with two different, average, but inherently constant energy levels. Such a control system can be compared with a two-point control. The disadvantage of this known system lies in its considerable dead time, since the control does not intervene quickly enough with impact-like counter-torques. In this way, there is a risk that the rotational speed will fall below a limit, which will result in a lag that can no longer be caught up.
The invention is based on the aim of specifying a control procedure and an arrangement which operate without noticeable dead times, which react to an impact-like counter-torque instantly with a corresponding increase of the driving power of the motor, and which nevertheless involve the smallest possible power consumption on a time average.